Much research and development effort has been expended recently in the quest to produce a charged-particle-beam (CPB) projection-exposure apparatus exhibiting both high resolution and high throughput. Contemporary CPB projection-exposure systems that provide high throughput are termed "batch-transfer" systems, with which a single die (one chip) or even multiple dies are simultaneously exposed onto the sensitive substrate. Unfortunately, batch-transfer systems have substantial problems. For example, a mask or reticle suitable for a batch-transfer system is typically very complex and preparing such a mask or reticle is extremely difficult. Another serious problem is that exposing an entire die in one "shot" requires an extremely large optical field in which substantial aberrations are typically present, especially in off-axis regions of the optical field. Aberrations in such large optical fields are extremely difficult to reduce satisfactorily to within the tight specifications normally required for accurate pattern transfer. Consequently, interest in batch-transfer systems has declined.
Another type of projection-exposure apparatus that has been under intensive development recently is the "divided" projection-exposure system in which the entire mask pattern is projection-exposed not simultaneously but rather in portions. The mask or reticle used with such a system is "segmented" into multiple "mask subfields" or other regions each defining a separate respective portion of the mask pattern. The mask subfields are individually projection-exposed sequentially onto corresponding regions on the sensitive substrate on which the resulting images are "stitched" together. Sequentially exposing all the mask subfields of a die results in reproduction of the entire mask pattern on the substrate. Since projection-exposure is performed subfield-by-subfield (or otherwise region-by-region), the optical field can be much smaller than the very large optical field required in a batch-transfer system. The smaller optical field required for divided projection-exposure generally allows better correction of aberrations (e.g., variability in focal plane, distortion, etc.) than batch-transfer systems. Thus, a divided projection-exposure system can perform projection-exposure with better resolution across an optically wide field, and more accurately and completely correct aberrations, compared to batch-transfer systems.
Certain conventional aspects of divided projection-exposure are shown in FIGS. 2(a)-2(b), in which item 100 is a mask (synonymous herein with "reticle"), item 100a is a mask subfield (as a representative region of the mask), item 100b is a boundary region situated between adjacent mask subfields 100a, item 110 is a sensitive substrate (e.g., semiconductor wafer) to the surface of which a resist has been applied, item 110a is a one-die (one chip) field on the sensitive substrate 110, item 110b is an individual "transfer subfield" (as a representative corresponding exposed region) on the sensitive substrate 110 corresponding to a respective mask subfield 100a, AX is the optical axis of the CPB optical system, EB is the charged particle beam, and CO is a crossover point of the CPB optical system.
The mask 100 comprises a membrane divided into multiple mask subfields 100a each comprising a respective portion of a mask pattern to be transferred to the sensitive substrate 110. The mask subfields 100a are delineated by boundary regions 100b that do not define any portion of the overall mask pattern. The mask 100 typically comprises a support grid (not shown, but typically situated on the downstream-facing surface of the mask 100) typically configured as intersecting struts coextensive with the boundary regions 100b. The support grid provides the membrane with thermal stability and physical strength.
If the sensitive substrate 110 is manufactured from a semiconductor material (e.g., silicon), the substrate typically has a profile as shown generally in FIG. 2(b). FIG. 2(b) shows a portion of the sensitive substrate 110 (area "Va").
In FIG. 2(a), a z-axis extends parallel to the optical axis AX of the CPB optical system, and the x- and y-axes extend parallel to the directions in which the mask subfields 100a are arrayed. The respective pattern portions defined by a row of subfields 100a are sequentially transferred by step-wise scanning the charged particle beam EB in the y-axis direction. To project a subsequent adjacent row of subfields, the mask 100 and sensitive substrate 110 are moved in opposite directions along the x-axis, as denoted by arrows Fm and Fw. Such scanning of the charged particle beam EB in the y-axis direction and movement of the mask 100 and substrate 110 in the x-axis direction are repeated until an entire die has been transferred. Then, the substrate 110 is moved to position the next die for exposure, and the mask 100 is exposed again subfield-by-subfield as described above.
The scanning sequence of mask subfields 100a and the transfer sequence onto the sensitive substrate 110 are denoted by arrows Am and Aw, respectively. The mask 100 and sensitive substrate 110 are moved in opposite directions along each of the x- and y-axes because these axes are inverted on the substrate 110 (relative to the mask 100) by passage of the charged particle beam EB through a pair of projection lenses (not shown, but situated between the mask 100 and substrate 110).
The boundary regions 100b separating the mask subfields 100a from one another do not define any portion of the mask pattern and thus are not transferred to the substrate 110. Hence, the positions of the transfer subfields 100b must be shifted sufficiently during projection-exposure to allow the transfer subfields to be "stitched" together on the substrate surface without any intervening gaps. To such end, the charged particle beam EB is laterally shifted, during projection-exposure of each mask subfield 100a, in the y-axis direction by an amount equal to the width Ly of the boundary region 100b. A similar type of correction is made in the x-axis direction by simultaneously shifting the mask 100 and substrate 110 by relative amounts taking into consideration the demagnification ratio of the combined projection lenses. Also, whenever transfer of a particular row of mask subfields 100a is completed and it is necessary to shift to the next row, the charged particle beam EB is shifted in the x-axis direction by an amount equal to the width Lx of the boundary region 100b. As a result, the transfer subfields 110b are formed contiguously on the substrate 110 without any intervening boundary regions or other gaps.
The segmented mask 100 typically includes a support grid (not shown in FIGS. 2(a)-2(b)) comprising intersecting struts that are coextensive with the boundary regions 100b. Thus, the support grid, along with the network of boundary regions 100b, separates the mask subfields 100a (or analogous regions) from one another. The support grid renders the mask 100 resistant to warping and thermal distortion including such warping and distortion caused by irradiation of the mask by the charged particle beam EB.
An example of a conventional divided projection-exposure apparatus is shown in FIG. 3. The FIG. 3 apparatus comprises a charged-particle source 1 that produces a charged particle beam 2; an illumination-lens assembly 3'; deflectors 4, 5; a projection-lens assembly 3; a contrast aperture 14; and a computer 16. The apparatus receives a mask 12 comprising multiple mask subfields 12a (or analogous regions) each defining a respective very small portion of the mask pattern (in FIG. 3, the mask subfield 12a corresponds with the mask subfield 100a shown in FIG. 2(a)). The mask subfields 12a are separated from one another by boundary regions 12b that define no portion of the mask pattern (the boundary region 12b corresponds with the boundary region 100b shown in FIG. 2(a)). The mask 12 further comprises a support grid 12c situated in the boundary regions 12b. The FIG. 3 apparatus is also usable with a substrate ("wafer") 13.
In FIG. 3, the electron beam 2 propagates generally along a z-axis extending vertically in the figure. X- and y-axis directions extend orthogonally with respect to the z-axis.
A conventional divided projection-exposure apparatus such as that of FIG. 3 typically also includes any of various deflectors (not shown) in association with the projection-lens assembly 3 that serve to provide, inter alia, automatic correction of deflection aberrations. Such deflectors are discussed below.
Referring further to FIG. 3, the charged particle beam 2 is emitted from the charged-particle source 1 and is shaped into a collimated beam by the illumination-lens assembly 3' for illumination of the selected region of the mask 12. So as to illuminate a single preselected mask subfield 12a, the charged particle beam 2 is appropriately deflected laterally by the deflectors 4, 5. For example, in FIG. 3, the cross-hatched area represents the charged particle beam irradiating a mask subfield 12a located on the right side of the mask. By changing the excitation current or voltage (depending upon whether the deflectors 4, 5 are electromagnetic or electrostatic, respectively) applied to the deflectors 4, 5, a more centrally located mask subfield 12a can be irradiated or, alternatively, a mask subfield 12a located on the left side of the mask can be irradiated.
The charged particle beam 2 that has passed through the selected region 12a of the mask passes through the projection-lens assembly 3 to form a pattern-feature image of the irradiated mask region on the wafer 13. The contrast aperture 14 is situated at a back-focal plane of the upstream projection lens 3 to block scattered charged particles from areas outside the pattern features.
To place the image of the irradiated mask region at the proper location on the wafer 13 and to reduce aberrations caused by deflection of the beam, the charged particle beam is appropriately deflected by deflectors (not shown) located downstream of the mask 12. Thus, the transfer subfields are formed contiguously and finely on the wafer 13 to form the mask pattern on the wafer 13.
CPB projection-optical apparatus (such as an apparatus shown generally in FIG. 3) are known that employ a dynamic compensator in association with the projection-optical system. In this regard, reference is made to Saitou et al., "A High-Speed, High-Precision Electron Beam Lithography System (Electron Optics)," J. Vac. Sci. Technol. B3(1):98-101, 1985. In this paper (specifically FIG. 1 of the paper), an objective lens (analogous to a projection-optical lens system) is situated between the mask and the substrate. The projection-optical lens system has associated therewith a dynamic compensator comprising a dynamic-focus coil and a stigmator coil. The dynamic-focus and stigmator coils are energized with respective excitation currents according to respective signals generated by a computer or the like. Each respective excitation current is a function of the beam position in the transverse (x-y) plane (i.e., of deflection position of the beam).
Reference is also made to U.S. Pat. No. 5,653,719 to Petric which discusses a different approach to reducing aberrations caused by deflection. Specifically, according to Petric, electron-beam optics are provided in which respective lens axes are shifted as a function of the respective beam position along the beam path, so as to follow the central ray of the beam as it is deflected through the respective lenses. Various deflectors in association with the projection-optical system are employed for this purpose.
Conventional methods and apparatus for correction of beam position and of aberrations exhibit generally satisfactory performance whenever the size of the mask subfields (or analogous irradiated regions of the mask) is small, i.e., at most several micrometers square. However, whenever the irradiated mask regions are larger (e.g., several hundreds of micrometers square), the conventional methods and apparatus do not exhibit satisfactory performance, especially in view of the increasing demands of pattern resolution required recently.
For example, in divided projection-exposure systems, aberrations other than main-field distortion, such as "hybrid blurs" and "hybrid distortions" (which are ignored in the FIG. 3 apparatus) become problematic. "Hybrid blurs" are aberrations the magnitudes of which depend on the aperture half-angle, position within the irradiated region, and the center position of the deflected region. "Hybrid distortions" are aberrations the magnitudes of which depend on the position within the irradiated region subfield and the center position of the deflected region.
In divided projection-exposure apparatus, hybrid blurs and hybrid distortion can have a magnitude of several tens of nm, and conventional methods as summarized above are inadequate for making the required corrections with respect to, inter alia, hybrid blur and hybrid distortion.